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A control register is a processor register which changes or controls the general behavior of a CPU or other digital device. Common tasks performed by control registers include interrupt control, switching the addressing mode, paging control, and coprocessor control.
The CR0 register is 32 bits long on the 386 and higher processors. On x86-64 processors in long mode, it (and the other control registers) is 64 bits long. CR0 has various control flags that modify the basic operation of the processor.
|31||PG||Paging||If 1, enable paging and use the CR3 register, else disable paging|
|30||CD||Cache disable||Globally enables/disable the memory cache|
|29||NW||Not-write through||Globally enables/disable write-back caching|
|18||AM||Alignment mask||Alignment check enabled if AM set, AC flag (in EFLAGS register) set, and privilege level is 3|
|16||WP||Write protect||Determines whether the CPU can write to pages marked read-only|
|5||NE||Numeric error||Enable internal x87 floating point error reporting when set, else enables PC style x87 error detection|
|4||ET||Extension type||On the 386, it allowed to specify whether the external math coprocessor was an 80287 or 80387|
|3||TS||Task switched||Allows saving x87 task context upon a task switch only after x87 instruction used|
|2||EM||Emulation||If set, no x87 floating point unit present, if clear, x87 FPU present|
|1||MP||Monitor co-processor||Controls interaction of WAIT/FWAIT instructions with TS flag in CR0|
|0||PE||Protected Mode Enable||If 1, system is in protected mode, else system is in real mode|
Contains a value called Page Fault Linear Address (PFLA). When a page fault occurs, the address the program attempted to access is stored in the CR2 register.
Used when virtual addressing is enabled, hence when the PG bit is set in CR0. CR3 enables the processor to translate linear addresses into physical addresses by locating the page directory and page tables for the current task. Typically, the upper 20 bits of CR3 become the page directory base register (PDBR), which stores the physical address of the first page directory entry.
Used in protected mode to control operations such as virtual-8086 support, enabling I/O breakpoints, page size extension and machine check exceptions.
|21||SMAP||Supervisor Mode Access Protection Enable||If set, access of data in a higher ring generates a fault|
|20||SMEP||Supervisor Mode Execution Protection Enable||If set, execution of code in a higher ring generates a fault|
|18||OSXSAVE||XSAVE and Processor Extended States Enable|
|17||PCIDE||PCID Enable||If set, enables process-context identifiers (PCIDs).|
|14||SMXE||Safer Mode Extensions Enable||see Trusted Execution Technology (TXT)|
|13||VMXE||Virtual Machine Extensions Enable||see Trusted Execution Technology (TXT)|
|10||OSXMMEXCPT||Operating System Support for Unmasked SIMD Floating-Point Exceptions||If set, enables unmasked SSE exceptions.|
|9||OSFXSR||Operating system support for FXSAVE and FXSTOR instructions||If set, enables SSE instructions and fast FPU save & restore|
|8||PCE||Performance-Monitoring Counter enable||If set, RDPMC can be executed at any privilege level, else RDPMC can only be used in ring 0.|
|7||PGE||Page Global Enabled||If set, address translations (PDE or PTE records) may be shared between address spaces.|
|6||MCE||Machine Check Exception||If set, enables machine check interrupts to occur.|
|5||PAE||Physical Address Extension||If set, changes page table layout to translate 32-bit virtual addresses into extended 36-bit physical addresses.|
|4||PSE||Page Size Extension||If unset, page size is 4 KiB, else page size is increased to 4 MiB (or 2 MiB with PAE set).|
|2||TSD||Time Stamp Disable||If set, RDTSC instruction can only be executed when in ring 0, otherwise RDTSC can be used at any privilege level.|
|1||PVI||Protected-mode Virtual Interrupts||If set, enables support for the virtual interrupt flag (VIF) in protected mode.|
|0||VME||Virtual 8086 Mode Extensions||If set, enables support for the virtual interrupt flag (VIF) in virtual-8086 mode.|
Extended Feature Enable Register (EFER) is a model-specific register added in the AMD K6 processor, to allow enabling the SYSCALL/SYSRET instruction, and later for entering and exiting long mode. This register becomes architectural in AMD64 and has been adopted by Intel.
|15||TCE (Translation Cache Extension)|
|14||FFXSR (Fast FXSAVE/FXRSTOR)|
|13||LMSLE (Long Mode Segment Limit Enable)|
|12||SVME (Secure Virtual Machine Enable)|
|11||NXE (No-Execute Enable)|
|10||LMA (Long Mode Active)|
|8||LME (Long Mode Enable)|
|0||SCE (System Call Extensions)|
The AMD64 architecture allows software to define up to 15 external interrupt-priority classes. Priority classes are numbered from 1 to 15, with priority-class 1 being the lowest and priority-class 15 the highest. CR8 uses the four low-order bits for specifying a task priority and the remaining 60 bits are reserved and must be written with zeros.
System software can use the TPR register to temporarily block low-priority interrupts from interrupting a high-priority task. This is accomplished by loading TPR with a value corresponding to the highest-priority interrupt that is to be blocked. For example, loading TPR with a value of 9 (1001b) blocks all interrupts with a priority class of 9 or less, while allowing all interrupts with a priority class of 10 or more to be recognized. Loading TPR with 0 enables all external interrupts. Loading TPR with 15 (1111b) disables all external interrupts.
The TPR is cleared to 0 on reset.
See also 
- Protected mode in x86 Assembly at Wikibooks
- General purpose registers
- Test register
- Debug register
- Flag byte
- Status register
- Anvin, H. Peter. "x86: Supervisor Mode Access Prevention". Retrieved 15 April 2013.
- "AMD64 Architecture Programmer?s Manual Volume 2: System Programming". AMD. Retrieved 3 April 2013.
- Intel IA-32 Architecture Software Developer’s Manuals
- AMD64 Architecture Tech Docs
- Cracking Open The Pentium III
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